1. Field of the Invention
The invention relates to a counter-current membrane module for liquid separations.
2. Description of the Prior Art
Separation processes are frequently used in the process industry, either as such or in combination with a subsequent or previous physical/chemical/biological treatment.
Membrane filtration processes can be an alternative for other separation processes, for example distillation, in particular in those cases where small differences in vapor pressure or azeotropic mixtures are present, or in separations in which one of the components is present in small quantities and has a higher boiling point than the other components.
For separating components in one liquid phase, reverse osmosis--hyperfiltration--can be used in many cases. For achieving high differences in concentration between feed and permeate, however, the usual "single-stage" processes are technically and economically not very suitable because of the high osmotic pressure difference between feed and permeate then occurring, due to which the effective pressure difference--the motive force behind permeate production--decreases so that, for achieving a certain level of production, capital expenditure as well as operating costs rise sharply.
Multistage installations are known in which the permeate from one process step is again repressurized in the following process step (cascade arrangement). This enables a good separation to be achieved, but at the expense of much higher energy consumption required for repeatedly repressurizing feed streams.
Counter-current membrane systems in which feed and permeate are consequently arranged running counter to each other are known as "Membrane rectification column", "Counter current reverse osmosis membrane column" (CCRO) and "Continuous" or "Reflux membrane column". Such an arrangement is known for gas separation, pervaporation and liquid separations, while a number of other uses such as concentrating organic substances from aqueous solutions and purification of waste water are also known.
Furthermore, a liquid-liquid separation in a reflux membrane column is known, whereby the operation of such a principle is experimentally demonstrated for an ethanol/water system.
Although it has been clearly recognized that, in the case of liquid-liquid separations in which high osmotic pressure differentials can occur, a counter-current arrangement might be superior to the available single-stage embodiment, their implementation has hardly been started. The advantage of counter-current would appear in particular on concentrating diluted solutions such as ethanol/water mixtures which are obtained, for example, by fermentation. Economic evaluations show that, on concentrating a 5% ethanol solution by means of a counter-current process, capital expenditure and energy costs can be reduced by a factor of at least two as compared to a single-stage process.
Consequently, there is a requirement for a counter-current membrane module in which a local transverse current is combined in one module with a general counter current, in order to achieve a higher mass transfer and less fouling.
The invention accordingly provides a counter-current membrane module for liquid separations consisting of a body comprising hollow fiber membranes in which the hollow fiber membranes are arranged perpendicularly to the longitudinal axis of the module and the channels in the hollow fibers are connected to a space present around the body which is furthermore surrounded by a shell, characterized in that the module comprises two or more transverse-current segments formed by seals between the body and the shell, wherein at least one fiber layer or fiber mat per segment has been placed.
In an advantageous embodiment of the invention the body and the shell are longitudinal. Furthermore the hollow fiber membranes can be incorporated in a fabric or mat.
By making feed and permeate flow in a general counter current and locally in a transverse current, an optimization of the motive force behind the separation process is achieved.
For reverse osmosis, for example, this means that the concentration difference across the membrane and, consequently, osmotic pressure differential, is kept to a minimum so that the effective pressure differential is a maximum. For dialysis, on the other hand, this achieves a maximum concentration gradient across the membrane and, consequently, a maximum motive force.
The invention is based in particular on mounting devices in the module for implementing transverse as well as counter currents.
The invention will now be explained in more detail with reference to the drawings and the description as follows.